Low pressure drop, high surface area ammonia oxidation catalyst

ABSTRACT

This invention is a catalytic element for use in the catalytic oxidation of ammonia. The element comprises a foraminous structure fabricated from a material consisting essentially of a metal selected from the group consisting of platinum, rhodium, palladium and alloys of mixtures thereof characterized by (a) a novel configuration whereby the initial product of the formula: curve the flat ratio (C/F) multiplied by mesh count in inches (N) and wire diameter (d w ), for said element is greater than at least about 0.08 and (b) where, for a given hydrogen throughput, the conversion efficiency is a function of the curve to flat ratio (C/F), wire diameter (d w ) and mesh count (N) combination and conversion efficiency is improved by increasing the mesh count (N) for a given wire diameter, increasing the wire diameter (d w ) for a given mesh count, and increasing the curve to flat ratio (C/F) to a ratio of above 1.0. Preferably the initial product of the formula is greater than 0.2, more preferably 0.9 and the curve to flat ratio (C/F) is above about π/2. The preferred initial product of the formula is in the range of from 0.08 to about 10 and more preferred from about 0.9 to about 10. The preferred element is woven gauze, knitted fabric, fibers and combinations thereof. The element can be in a series of said elements as a plurality of screens.

FIELD OF THE INVENTION

This invention relates to a low pressure drop, high surface area ammoniaoxidation catalyst system and method.

BACKGROUND OF THE INVENTION

Nitric acid is produced commercially by passing ammonia and air across aflat gauze woven from platinum-rhodium-palladium alloy wire. Theammonia, initially, is converted to nitric oxide over the precious metalgauze, and the nitric oxide is, subsequently, oxidized and absorbed toform nitric acid. The conversion efficiency of ammonia to nitric oxideis a function of temperature, pressure, velocity of gas stream, volumeof catalyst, and purity of the ammonia and air streams. The ammoniaoxidation to nitric oxide generates a large exotherm and raises thetemperature of the catalyst in the rang of 810° C. to 960° C. During theammonia oxidation process some of the precious metal is volatilized fromthe surface of the gauze wire. The rate of loss is dependent on thetemperature, pressure, and flow rate of gases across the catalystsurface. The cost of the precious metal lost from the ammonia oxidationcatalyst is significant part of the cost of operating a nitric acidplant.

In nitric acid production, the catalyst pack consists of 3 to 50 sheetsof flat woven gauze. The conventional flat woven gauze is typically madewith 80 mesh per inch and 0.003" wire. However, UK patent GB 2062486Bdiscloses the use of a system where the diameter of the wire is reducedfrom the front to the back of the flat woven gauze pack resulting inmaximum conversion efficiency with minimum precious metal content andtherefore also minimum metal loss from the catalyst. Other recentdisclosures include U.S. Pat. No. 4,863,893 which claims improvedcatalyst "light-off" by the use of a high surface area per unit area ofcatalyst by the deposition of fine platinum particles onto the surfaceof the flat woven gauze. U.S. Pat. No. 4,863,893 is a variation of thetechnology of U.S. Pat. No. 3,470,019 which had a different method ofdeposition. Patent application EP O 364 153 A1 claims the use of a flatknitted fabric of precious metal for the oxidation of ammonia to nitricoxide. Recently, a flat woven gauze of 70 mesh, 0.003" wire diameter wasintroduced to the market place.

The catalyst pack of elements or sheets of gauze of precious metals andits use to catalyze ammonia is disclosed in U.S. Pat. No. 4,412,859,U.S. Pat. No. 4,497,657 and U.S. Pat. No. 4,526,614, all of which arehereby incorporated by reference, in toto.

By curve to flat ratio is meant the ratio of that portion of an elementof a catalyst (a sheet or gauze etc.) that is not in the base plane ofthe element to the portion that is in that plane. When curved, it is theratio of curved section to the flat section.

SUMMARY OF THE INVENTION

This invention is a catalytic element for use in the catalytic oxidationof ammonia. The element comprises a foraminous structure fabricated froma material consisting essentially of a metal selected from the groupconsisting of platinum, rhodium, palladium and alloys of mixturesthereof characterized by (a) a novel configuration whereby the initialproduct of the formula: curved the flat ratio (C/F) multiplied by meshsize (N) and wire diameter (d_(w)), for said element is greater than atleast about 0.08 and (b) wherefore a given nitrogen throughput, theconversion efficiency is a function of the curve to flat ratio (C/F),wire diameter (d_(w)) and mesh size (N) combination and conversionefficiency is improved by increasing the mesh size (N) for a given wirediameter, increasing the wire diameter (d_(w)) for a given mesh size,and increasing the curve to flat ratio (C/F) to a ratio of above 1.0.Preferably the initial product of the formula is greater than 0.2 andthe curve to flat ratio (C/F) is above 1.00 (most preferably π/2). Thepreferred initial product of the formula is in the range of from 0.08 toabout 10 and more preferred from about 0.2 to about 10. The preferredelement is woven gauze, knitted fabric, fibers and combinations thereof.Preferably the element is in a series of said elements as a plurality ofscreens.

Preferably the element consists of platinum or essentially of platinumalloyed with one or more metals selected from the group consisting ofnickel, cobalt, palladium, ruthenium, iridium, gold, silver and copper.The preferred element contains platinum present in an amount of at leastof about 70% by weight.

The preferred embodiment of this invention is an element wherein (C/F)is in the range of from above 1.0 to about 4, N is the range of from 40to 120, d. is in the range of from about 0.001 to 0.018 and theirrespective values are such that the initial product of the formula isgreater than at least 0.08, more preferably C/F is from above 1.00 (mostpreferably π/2) to about 4 and the product of the formula is greaterthan 0.2. In another embodiment, the initial product is in the range offrom 0.08 to about 10, more preferably from about 0.9 to about 8.

In a preferred embodiment of this invention, the C/F ratio is achievedby means of forming the element into a pleat-like configuration. Thepleats are preferably parallel. Alternatively, the ratio can be achievedby means of pleats in concentric patterns. Such patterns can be circles,parallel lines or polygons. The ratio can also be achieved means ofintersecting patterns resulting in the waffle-like pattern. Thewaffle-like pattern can be regular comprising straight lines or curvedlines, or both. The waffle-like pattern could also be random with eitherstraight or curved lines, or both. In another embodiment the C/F ratiocan be achieved by means of shaped depressions on the surface of theelement.

The method of this invention is a method for catalytic oxidation ofammonia at temperatures above 850° C. which comprises using as catalysta foraminate element fabricated from metal consisting essentially of ametal from the group consisting of platinum, rhodium, palladium andalloys of mixtures thereof characterized by (a) a novel configurationwhereby the initial product of the formula: curve to flat ratio (C/F)multiplied by mesh size (N) and wire diameter (d_(w)) for the element,is greater than at least about 0.08 and (b) where, for a given nitrogenthroughput, the conversion efficiency is a function of the curve to flatratio (C/F), wire diameter (d_(w)) and mesh size (N) combination andconversion efficiency is improved by increasing the mesh size (N) at agiven wire diameter, increasing the wire diameter (d_(w)) at a givenmesh size, and increasing the curve to flat ratio (C/F) to a ratio ofabove 1.0. Preferably the formula product is greater than about 0.9 andthe C/F ratio is above about π/2. Preferably the product of the formulais in the range of from about 0.08 to about 10, more preferably to about8, most preferably about 0.20 to about 8.

The method preferably uses elements of woven gauze, knitted fabric,fibers or combinations thereof. Preferably the elements are in aplurality screens. The preferred method of this invention uses elementsconsisting essentially of platinum or platinum alloys of one or moremetals selected from the group consisting of nickel, cobalt, palladium,rhodium, ruthenium, iridium, gold, silver and copper. Preferablyplatinum is present in the amount of at least about 70% by weight.

The preferred method of this invention is where (C/F) is in a range offrom about greater than 1.0 to about 4, N is in the range of from about40 to about 120, d_(w) is in the range of from about 0.001 to about0.018 and their respective values are such that the initial product ofthe formula is greater than about 0.08. Most preferably C/F is in arange of from about π/2 to about 4 and the initial product of theformula is greater than at least of about 0.9. Preferably the initialproduct of the formula is in the range of from about 0.08 to 10 and mostpreferably from about 0.2 to 8. Another embodiment of this inventionwhen using multiple elements of this invention is modulating the initialproduct of the formula across the elements. The product can be modulatedto change from higher at those elements initially contacted by ammoniato lower where the ammonia last contacts the elements. Or vice versa,from lower as the elements initially contacts the ammonia to higherwhere the ammonia last contacts the elements. The product of the formulacan change by a recognizable pattern across the system, preferably apattern described a variety of mathematical functions, most preferablythe mathematical function selected from the groups consisting of linear,parabolic, hyperbolic, step, sinusoidal and combinations thereof. Thismodulation described above is useful both in the method of thisinvention and in the catalyst system using elements described by thisinvention.

The modulation may be by varying any one of the product multipliers C/F,d or N independently and/or in coordination, C/F N and d_(w) may beconstant or variable both screen-to-screen and/or across each screen andindependently varied across the warp and/or across the weft of the wovenor knitted screen. Thus, when variable within the screen, the definitionof C/F, N and d_(w) in this invention shall all be average numbers foreach individual screen. For example, varying both N and d_(w), theformula would be calculated as follows.

Warp: 50 wires per inch @0.0058 inch wire diameter.

Weft: 30 wires per inch @0.0097 inch wire diameter.

Developed Curve to Flat: 2.61 ##EQU1##

This invention provides the following benefits.

1. By using a C/F ratio over 1, formerly flat screens are madethree-dimensional, thereby increasing the surface area for greatlyimproved catalytic activity of the catalyst element. For example, usingtypical 80 mesh, 3 mil (0.003") screen, surface area per cross sectionalarea of the reactor can be increased threefold from 1.5 units at C/Fequals 1, i.e. flat, to 4.5 units at C/F equals 3.

2. The higher surface area, per screen, made possible by increasing C/Fratio permits fewer elements or screens in the system and lower pressuredifference across the total system used. This in turn permits higherthroughput rates of ammonia through the system, i.e., more tons per dayconversion of ammonia from the same equipment.

3. The conversion of ammonia over the catalyst elements (screens) isalso improved by the higher surface area of each element, more ammonia,per ton of ammonia throughput, is converted to nitric acid through thesame equipment.

4. By another embodiment of this invention, namely, modulation of thefactors determining surface area, either individually or by the formula,C/F (N) dw, the catalyst surface area can by tailored to be maximum atthe point in the system (screen pack) it is most effective andefficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a computer model generated set of curves projecting how thelow pressure drop catalyst system would lower pressure drop and increaseresidence time in 100 T/m² /day plant using 30 sheets of 3 mil (0.003")80 mesh sheets (elements) at varying C/F ratios.

FIG. 2 is a prior art curve of actual values of conversion efficiency atvarious residence time (sec.)/number of gauzes (elements).

FIG. 3 is a set of curves showing improved residence time at higher C/F(curve to flat) ratios.

FIG. 4 is a curve showing conversion efficiency at a 100 ton nitric acidplant using 30 sheets (elements) of catalyst at various C/F ratios.

FIG. 5 is a second curve showing conversion efficiency but with 20sheets (elements).

DISCUSSION

FIG. 1 shows that pressure drop across the system would be significantlylower, especially above C/F of π/2 and residence times can besignificantly increased at higher C/F ratios than 1. For example,residence time is doubled between C/F values of about 1.1 and about 1.6.

FIG. 2 shows the importance of residence time in a prior art system atthe conditions shown. The curve shows that conversion efficiencyincreases with residence time to an optimum at a residence time of about7×10⁻⁴ seconds with about 50 gauzes (elements or sheets).

Thus in a preferred embodiment of this invention, the optimum residencetime would be found for the commercial plant in which the system will beused. Then the curves similar to FIG. 1 would be generated using thenitrogen loading, number of sheets in the catalyst system and thedesired mesh and wire diameter of each sheet desired. A series of suchcurves could be generated for various desired wire diameters and meshes.Then the computer generated residence time and pressure drop curveswould make possible picking a C/F ratio for the actual commercialapplication. For a crude example, for the 100 T/m² /day ammoniaconversion plant using a 30 sheet (elements) catalyst system of 3 milwire diameter, 80 mesh catalyst (90% Pt: 10% Rh) would pick the highestC/F ratio available (or practicable) using FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

A computer model of the system of this invention is used to predictperformance of various catalyst systems is various commercial plantsituations.

EXAMPLE 1

Nitric Acid Production: 100 Tons per Day

Nitrogen Loading: 116 tons N/meter² /day

Inlet Temperature: 250 degrees Celsius

Number of Woven Sheets: 30

The current residence time for a flat system is 4×10⁻⁴ seconds with aconversion efficiency of 96% (see FIG. 4 for 30 sheets).

It has been found through experimentation that the optimum efficiency isobtained at a residence time of 6.0×10⁻⁴ seconds. To obtain this optimumresidence time, the developed curve to flat ratio must be increased from1 to 1.5. This will increase the conversion of ammonia from 96% to 99%at the same operating conditions.

EXAMPLE 2

The operating conditions are the same as the preceding example.

The nitric acid producer is not interested in increasing conversionefficiency, but would like to lower the precious metal loading in hisreactor. This can be achieved by arbitrarily reducing the number ofsheets being used. For the number of sheets equal to 20, the curve toflat ratio must be increased from 1 to 1.5 to obtain a conversionefficiency of 96% (see FIG. 5 for 20 sheets).

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,the invention is not limited to the disclosed embodiment but, on thecontrary, is intended to cover various modifications and equivalentsincluded within the spirit and scope of the following claims.

We claim:
 1. The method for the catalytic oxidation of ammonia attemperatures above 850° C. which comprises using as catalyst aforaminate element fabricated from metal consisting essentially of ametal selected from the group consisting of platinum, rhodium, palladiumand alloys of mixtures thereof characterized by (a) a novelconfiguration whereby the initial product of the formula: curve to flatratio (C/F) multiplied by mesh count per inch (N) and wire diameter ininches (d_(w)) for said element is greater than at least about 0.08 butless than about 10 and (b) where, for a given ammonia throughput, theconversion efficiency is a function of the curve to flat ratio (C/F),wire diameter (d_(w)) and mesh count (N) combination and conversionefficiency is improved by increasing the mesh count (N) at a given wirediameter, increasing the wire diameter (d_(w)) at a given mesh count,and increasing the curve to flat ratio (C/F) to a ratio of above 1.0. 2.The method of claim 1 which said C/F ratio is above about π/2.
 3. Themethod according to claim 1 wherein said product has a range of about0.2 to about
 8. 4. The method according to claim 1 wherein said elementis a woven gauze, knitted fabric, fibers or combinations thereof.
 5. Themethod according to claim 1 wherein said element is in a plurality ofscreens.
 6. The method according to claim 1 wherein said element consistessentially of platinum alloyed with one or more metals selected fromthe group consisting of nickel, cobalt, palladium, rhodium, ruthenium,iridium, gold, silver and copper.
 7. The method according to claim 6wherein said platinum is present in an amount of at least about 70% byweight.
 8. The method according to claim 1 wherein C/F is a range offrom greater than 1.0 to about 4, N is in the range of from about 40 toabout 120, d_(w) is in the range of from about 0.001 to about 0.018 andtheir respective values are such that the initial product of saidformula is greater than at least about 0.08.
 9. The method according toclaim 1 wherein C/F is in a range of from about π/2 to about 4, N is inthe range of from about 40 to about 120, d_(w) is in the range of fromabout 0.001 to about 0.018 and their respective values are such that theinitial product of said formula is greater than at least about 0.2. 10.The method according to claim 1 wherein the initial product of saidformula is in the range of from about 0.2 to
 8. 11. The method of claim5 wherein said initial product of said formula is modulated across theelements, element to element.
 12. The method of claim 11 wherein saidproduct changes from a higher number at the elements initially contactedby ammonia to a lower number where said ammonia last contacts theelements.
 13. The method of claim 11 wherein said product changes from alower number at the elements initially contacted by ammonia to a highernumber where said ammonia last contacts the elements.
 14. The method ofclaim 11 wherein said product changes by a mathematical pattern ofchange across the system elements.
 15. The method of claim 14 whereinsaid mathematical patterns is selected from the group consisting oflinear, parabolic, hyperbolic, step, sinusoidal and combinationsthereof.
 16. The method of claim 11 wherein said C/F ratio value ismodulated.
 17. The method of claim 16 wherein said ratio changes from ahigher number at the elements initially contacted by ammonia to a lowernumber when said ammonia last contacts the elements.
 18. The method ofclaim 16 wherein said ratio changes from a lower number at the elementsinitially contacted by ammonia to a higher number where said ammonialast contacts the elements.
 19. The method of claim 16 wherein saidratio changes by a mathematical pattern of change across the elements.20. The method of claim 19 wherein said mathematical patterns isselected from the group consisting of linear, parabolic, hyperbolic,step, sinusoidal and combinations thereof.